The present invention relates to fiber-optic communications networks, and more particularly, to Raman fiber amplifiers for use in optical communications networks.
In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers are used in such networks to amplify optical signals that have been subject to attenuation over fiber-optic links. A typical amplifier may include erbium-doped fiber amplifier components that are pumped with diode lasers. Raman amplifiers have also been used that provide optical gain through stimulated Raman scattering.
The gain spectrum of a Raman-pumped fiber amplifier that is pumped at a single wavelength is not flat. Unless the gain spectrum of the Raman amplifier is flattened, different wavelengths of light will be amplified by different amounts. This is undesirable, particularly in arrangements in which many amplifiers are cascaded in a communications link.
One way in which to flatten the gain spectrum of a Raman amplifier is to use a gain equalization filter. A gain equalization filter may be inserted in the optical path of the Raman amplifier. The gain equalization filter introduces losses in the portions of the spectrum in which the Raman gain is greatest, thereby flattening the Raman amplifier gain spectrum. However, the use of a gain equalization filter to flatten the Raman gain spectrum may increase the noise figure of the amplifier.
Another way in which to flatten the gain spectrum of a Raman amplifier is to use multiple pump wavelengths. The pump wavelengths may be selected so that the Raman gain peaks produced by each pump wavelength overlap. The overlapping gain peaks produce an overall gain spectrum that is relatively flat, but this may not be the most efficient way in which to generate a desired gain spectrum.
It is an object of the present invention to provide a Raman amplifier that reduces or eliminates the need for gain equalization filters.
This and other objects of the invention are accomplished in accordance with the present invention by providing Raman fiber amplifiers in which multiple pump wavelengths are used. An amplifier may be used to amplify optical signal channels over a range of wavelengths. Some of the pump wavelengths may be selected to create Raman gain in this range. Other pump wavelengths may be selected to create Raman loss in this range. Different pump powers may be used at each pump wavelength. Using both Raman gain and Raman loss contributions to achieve a given gain spectrum may be more efficient than using pumping schemes that only use Raman gain contributions.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.